1. Field of the Invention
The present invention relates to a heat resistant alloy for use as a material of engine valves mainly used in exhaust engine valves in automobiles.
2. Description of the Related Art
Conventionally, a Fe-based alloy (heat resistant steel) such as SUH11 and SUH35 has been widely used for automotive engine valves, but NCF751 (Ni-15.5Cr-1Nb-2.3Ti-1.2Al-7Fe in mass percent) of a Ni-base superalloy has been used in accordance with an increase of the use temperature.
However, NCF751 is more expensive than the Fe-based alloy because NCF751 includes a Ni content as high as about 70%. For this reason, a resource-saving type alloy having high-temperature strength and structural stability after a long-time exposure at high temperature, close to those of NCF751, has been developed. As a result, for instance, a Fe-based heat-resistant alloy of which the Ni content is decreased to 30 to 35 mass %, is disclosed in Japanese Laid Open Patent JP H09-279309-A2, a Fe-based heat-resistant alloy of which the Ni content is decreased to 30 to 49 mass %, in JP H07-109539-A2, and a Fe-based heat-resistant alloy of which the Ni content is decreased to 35 to 45 mass %, in JP H07-332035-A2. In addition, high-Ni heat-resistant alloys having more excellent high-temperature strength than NCF751 are disclosed in JP H07-216482-A2, JP H11-229059-A2 and others.
In recent years, against the background of global environmental problems, a more improved high-temperature strength which even NCF751 can not satisfy has been partly required for a valve material for the purpose of further increasing an efficiency of an engine. On the other hand, the resource saving and cost reduction of a component are expected for strengthening cost competitiveness in a globalized market.
The above-described alloys disclosed in JP H09-279309-A2, JP H07-109539-A2 and JP H07-332035-A2 contain 49 mass % or less Ni, so that they are excellent in terms of resource saving and cost reduction, but do not have more excellent high-temperature strength than NCF751. In addition, an alloy disclosed in JP H07-216482-A2 has more excellent high-temperature strength than NCF751 has, but is practically considered as an alloy containing more than 60 mass % Ni, so that it is insufficient in the respect of resource saving and cost reduction.
An alloy disclosed in JP H11-229059-A2 proposed by one of the present applicants, which has excellent high-temperature strength and a low cost at the same time, which has not been achieved in a conventional alloy, contains 50 to 60 mass % Ni and has a lower cost and more excellent high-temperature strength than NCF751 has. However, it was elucidated that the alloy has insufficient structural stability at a high temperature, and consequently has the possibility of decreasing toughness by being heated for a long time when used as a valve.